Drop unused ci builds index

Ref: #271620 (closed)

What does this MR do and why?

This MR drops our index_ci_builds_on_user_id_and_created_at_and_type_eq_ci_build which does not seem to be used according to Thanos.

Screenshots or screen recordings

Database

Migrations output:

main: == 20220629093102 DropUserCreatedAtFromCiBuilds: migrating ====================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- indexes(:ci_builds)
main:    -> 0.0082s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0001s
main: -- remove_index(:ci_builds, {:algorithm=>:concurrently, :name=>"index_ci_builds_on_user_id_and_created_at_and_type_eq_ci_build"})
main:    -> 0.0034s
main: -- execute("RESET statement_timeout")
main:    -> 0.0001s
main: == 20220629093102 DropUserCreatedAtFromCiBuilds: migrated (0.0175s) ===========

ci: == 20220629093102 DropUserCreatedAtFromCiBuilds: migrating ====================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- indexes(:ci_builds)
ci:    -> 0.0070s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0001s
ci: -- remove_index(:ci_builds, {:algorithm=>:concurrently, :name=>"index_ci_builds_on_user_id_and_created_at_and_type_eq_ci_build"})
ci:    -> 0.0029s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0001s
ci: == 20220629093102 DropUserCreatedAtFromCiBuilds: migrated (0.0124s) ===========


--------------------------------------------

main: == 20220629093102 DropUserCreatedAtFromCiBuilds: reverting ====================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- index_exists?(:ci_builds, [:user_id, :created_at], {:where=>"((type)::text = 'Ci::Build'::text)", :name=>"index_ci_builds_on_user_id_and_created_at_and_type_eq_ci_build", :algorithm=>:concurrently})
main:    -> 0.0092s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0001s
main: -- add_index(:ci_builds, [:user_id, :created_at], {:where=>"((type)::text = 'Ci::Build'::text)", :name=>"index_ci_builds_on_user_id_and_created_at_and_type_eq_ci_build", :algorithm=>:concurrently})
main:    -> 0.0045s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20220629093102 DropUserCreatedAtFromCiBuilds: reverted (0.0183s) ===========


ci: == 20220629093102 DropUserCreatedAtFromCiBuilds: reverting ====================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- index_exists?(:ci_builds, [:user_id, :created_at], {:where=>"((type)::text = 'Ci::Build'::text)", :name=>"index_ci_builds_on_user_id_and_created_at_and_type_eq_ci_build", :algorithm=>:concurrently})
ci:    -> 0.0090s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0002s
ci: -- add_index(:ci_builds, [:user_id, :created_at], {:where=>"((type)::text = 'Ci::Build'::text)", :name=>"index_ci_builds_on_user_id_and_created_at_and_type_eq_ci_build", :algorithm=>:concurrently})
ci:    -> 0.0054s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0002s
ci: == 20220629093102 DropUserCreatedAtFromCiBuilds: reverted (0.0189s) ===========

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